Conventionally, an engine and an automatic transmission are coupled via a fluid coupling (torque converter). ATF (Automatic Transmission Fluid) supplied to the torque converter is suctioned by an oil pump from an oil pan provided beneath the automatic transmission. For example at very low temperature, the oil level lowers with a decrease in volume of the ATF, and the amount of the oil returned to the oil pan decreases with an increase in viscosity, in which case the air may be suctioned together with the ATF.
Japanese Patent Laying-Open No. 02-271147 discloses an oil storing device of an automatic transmission capable of suppressing intake of the air. In the oil storing device of an automatic transmission described in Japanese Patent Laying-Open No. 02-271147, oil for use in controlling the oil pressure is stored in an oil reservoir beneath the automatic transmission, and an oil strainer is provided such that the suction port is located below the oil level. The oil storing device includes a variable capacity chamber of which capacity can be changed in accordance with the pressure of the fluid supplied inside, which chamber is provided in the oil reservoir such that it expands/contracts in the direction orthogonal to the sidewall surface of the oil reservoir. The pressure of the fluid in the variable capacity chamber is adjustable by means of a fluid pressure control device provided outside the oil reservoir. The fluid pressure control device is configured to increase the fluid pressure when it is necessary to raise the oil level. When the engine is started in the low temperature state, blow-by gas is introduced into the variable capacity chamber, whereby the variable capacity chamber expands.
According to the oil storing device of an automatic transmission described in this publication, the variable capacity chamber is provided to face the inner wall surface of the oil reservoir and made to expand in the direction orthogonal to the sidewall surface as necessary. At the time when the air is likely to be taken in through the suction port of the oil strainer as in the state of low temperature, in the case of rapid acceleration/deceleration, or in the case of sharp turn or the like, the variable capacity chamber is expanded to push aside the oil, to thereby raise the oil level. This can suppress intake of the air.
In the oil storing device of an automatic transmission described in Japanese Patent Laying-Open No. 02-271147, however, the variable capacity chamber is expanded by the blow-by gas. As such, in the case where the amount of the blow-by gas is insufficient, immediately after startup of the engine for example, there may be a case where the oil level cannot be raised sufficiently, in which case the air may be taken in. If the air thus taken in is fed to the fluid coupling provided between the engine and the automatic transmission, the air may accumulate inside the fluid coupling, leading to degradation in motive power transfer capability of the fluid coupling.